This invention relates generally to trigger circuits, and more particularly to a circuit for controlling the frequency and gain characteristics of the trigger signal.
Various trigger circuits are widely utilized for operation of triggerable elements or circuits such as, for example, sweep ramp generators of oscilloscopes. To provide versatility of such instruments, trigger circuits usually include a trigger coupling circuit for selecting the desired frequency response or signal attenuation.
One example of a conventional trigger circuit is shown in FIG. 1. In this particular example, three triggering signals, which suitably, may represent channel 1 (CH 1) and channel 2 (CH 2) input signals to a dual channel oscilloscope and an external (EXT) triggering signal, are applied to input connectors 10a through 10c. Any one of such trigger signals is selected by selection switch 12a through 12d for selecting alternatively any one of the CH 1, CH 2, EXT or EXT trigger signals divided by RC attenuator 14. The selected trigger signal is, then, supplied to the input of input amplifier 18 by way of coupling circuit 16 including four switches 20a through 20d, capacitors 22 through 26 and resistors 28 and 30. Switches 20a through 20d select respectively high frequency reject (HF REJ), AC coupling (AC), low frequency reject (LF REJ) and DC coupling (DC) modes. Input attenuator 18 includes source follower field effect transistor (FET) 32, current source FET 34 and input resistor 36. The output of input amplifier 18 is then compared with controllable trigger level from potentiometer 40 by differential comparator 38 to provide an output trigger signal from output terminal 42.
In operation, attenuator 14 attenuates the EXT trigger signal, for example, by the factor of 10 over the entire frequency range. AC coupling capacitor 22 has a relatively large capacitance, e.g. 0.027 .mu.F to eliminate DC and very low frequency signal components when the AC coupling mode to chosen. When switch 20a is closed to choose HF REJ coupling mode, resistor 28 and capacitor 26 form a low pass filter with upper cut-off frequency of about 30 KHz. Capacitor 24 is a very small capacitance, e.g. 100 PF, to reject low frequency components up to about 50 KHz.
Disadvantages of such prior art trigger circuits include the use of many switches in series with the high frequency signal paths, thereby making the circuit arrangement very critical to high frequency operation, especially when implemented in integrated circuit form. In this example, the DC, AC and LF REJ paths must be high frequency signal paths over the entire bandwidth (100 MHz or higher).
It is therefore, a feature of this invention to provide an improved trigger circuit including both high and low frequency signal amplifiers.
It is another feature of this invention to provide a trigger circuit having fewer high frequency signal paths.
It is still another feature of this invention to provide a trigger circuit capable of selecting various trigger coupling frequency responses without using a large coupling capacitor in series with the high frequency signal path.
It is an additional feature of this invention to provide a trigger circuit capable of attenuating the input trigger signal.
These and other features as well as advantages and operation of this invention will become apparent to one skilled in the art upon a reading of the following description when taken in conjunction with the accompanying drawings.